Thought I would share with you some thoughts on radius dishes. There are many builders who don’t use them…but if you are starting up (as myself) and want one, they are very costly to buy.
However, luckily for us, they’re dead simple to make. Mine took about an hour to make. You will require two pieces of MDF ($13 at home depot), 16 screws (or 24 depending on the size of your dish), a drill, a countersink bit ($10), and some wood glue.
First step is to determine the radius of your dish, and the size of the dish. I made mine 3’x3′ and 15 foot radius (for the back). I’ll make another with a 25′ radius tomorrow for the top.
I did not make this image! I don’t know who did. You can find the build process though by somebody (also not the person who originally came up with this technique) here. Makes very little mess (unlike the router method).
You simply start by marking out a circle on your MDF and cutting it out on the bandsaw at the size you want your dish. Cut out pieces as done in the thread linked above at the thickness (h) and calculate, using the formulas above, the placement distance from the center (r) given the radius (R). Some easy math and you are off and away toward completing the dish.
This part is important – on both the thin piece of MDF (1/4″ used for top of dish) and the base (1/2″ MDF), mark out a cross which intersects the center point of the circle. The two lines forming the cross should intersect at exactly 90 degrees. Make another cross like this one on each dish rotates 45 degrees, so you have the dish essentially having eight lines protruding from the center point (on the base and the top). Draw a circle at radius ‘r’ (depending on how high your spacers are this value will change). I used 3 mm and 8 mm spacers.
The points where these lines intersect the circle at radius ‘r’ is where your eight spacers are placed; drill a pilot hole first at the intersection point on both the top and base, then countersink only the top (1/4″ MDF) piece.
Set it up so you have either two or three circles at different radii with spacers of increasing height toward the edge of the base. Place the pieces with the wood glue (in link above he used CA glue, not super important though) along the pilot holes (see thread linked – the spacers have a hole, the spacer is placed so the hole in the spacer lines up with the pilot hole on the base of the dish).
Once all spacers are in place, line up the 1/4″ top onto the base of the dish. Screw the top down – the screws should pull the 1\4″ MDF flush with the spacers, forming a dish shape with the correct curvature. The hole in the spacers is to allow the screws to screw down without cracking the spacers.
The author of the thread where I found this method (again, original post is here ) ended up with this as the finished product:
Finished Radius Dish.
A simple but very effective method! Thanks to whoever created this, and thanks to the original poster on Ukulele Underground (Dave Higham). I’ll post pics of my dish later, as well as the Cocobolo OM guitar I’m working on (sides are glued up now, next step is the back radius, then I’ll start on the top).